1. Field of the Invention
The invention relates to superconductive switching elements and gates and more particularly to improved Josephson tunnel junction devices for performing such functions.
2. Description of the Prior Art
Superconductive memory and logic circuits are well known in the art which utilize the Josephson tunnel junction as an active switching or gating element. The Josephson tunnel junction device may also be employed as a magnetic field sensor, as in magnetometers and in electromagnetic wave detectors. The Josephson junction device normally comprises two adjacent layers of superconductive material with a barrier layer therebetween; Josephson tunneling current flows from one superconductive layer to the other through the barrier by use of the Josephson tunneling effect. With the superconductive layers connected into a superconductive loop and with control lines disposed adjacent the junction, the Josephson zero-voltage direct current flowing through the device is controlled so as to provide the necessary current steering and control functions in associated circuits.
A particular Josephson junction device, as well as problems attendant prior art devices, is described in U.S. Pat. No. 4,176,365, entitled "Josephson Tunnel Junction Device with Hydrogenated Amorphous Silicon, Germanium or Silicon-Germanium Alloy Tunneling Barrier", issued Nov. 27, 1979 in the name of Harry Kroger and assigned to Sperry Corporation.
As discussed in the Kroger patent, it is desirable to provide Josephson junction devices capable of supporting large critical current densities while still utilizing relatively thick barrier layers so as to provide structural rigidity and reliability and to decrease device capacitance. Oxide insulator tunneling barrier layers have generally been unsatisfactory in the past in that only relatively thin and consequently fragile insulator barriers could pass large Josephson direct currents, the thin barriers resulting in Josephson devices with undesirably high capacitance. The device of the aforementioned Kroger patent utilizes appropriately doped amorphous hydrogenated silicon, germanium, or alloys thereof as the barrier layer material, which arrangement successfully provides an unusually high current density through a suitably thick tunneling barrier layer.
Also providing readily reproducible high current density, small Josephson superconducting loops utilizing small area tunnel junctions and having barriers of sufficient thickness are the devices of the Harry Kroger U.S. patent application Ser. No. 23,487, filed Mar. 23, 1979 for a "Josephson Tunnel Junction Device with Polycrystalline Silicon, Germanium, or Silicon-Germanium Alloy Tunneling Barrier". As discussed also in U.S. Pat. No. 4,176,365, it is desirable to utilize refractory superconductive metals in fabricating Josephson junction devices. Generally, non-refractory materials have been used in prior art devices. Although non-refractory superconductive metals such as lead, indium, and tin and alloys thereof exhibit suitable superconductive properties, these materials cause difficulties when fabricating superconducting integrated circuits using fine line connections. Printed circuit fabrication techniques that tend partially to ameliorate these difficulties tend to be more complicated than more conventional fabrication techniques that presently produce high yield integrated circuits of a more conventional variety, such as in ordinary semiconductor circuits. In place of the soft, relatively low melting point superconductive metals mentioned, aluminum has been utilized as the superconductor to alleviate the problem, but aluminum has a significantly lower superconductive transition temperature than, for example, lead and therefore it requires much more energy to refrigerate the devices.
Patent application Ser. No. 23,487 now U.S. Pat. No. 4,220,959, additionally concerns an improvement like that of the earlier Kroger application that overcomes additional disadvantages of the prior art. Again, the invention is a Josephson junction device comprising first and second layers of superconductive material superposed one with respect to the other, the lower one of the layers comprising a refractory compound superconductive metal. A barrier layer is introduced between the first and second superconductive layers whereby Josephson tunneling current flows between the superconductive layers, the barrier layer comprising polycrystalline semiconductor material selected from the group consisting of silicon, germanium, and alloys thereof. Particularly, the lower superconductive layer is comprised of niobium nitride and the barrier layer is deposited thereon by chemical vapor deposition. The semiconductor barrier layer may be intrinsic or may be precisely doped to demonstrate either an n or p-type nature.